Trying to determine required force on steam turbine linear actuator

Hi! I´ve been lurking forever in this forum and finally I have an interesting question to ask and participate.
I´m a bit stuck, so any input would be highly appreciated!
I´m working on replacing a steam turbine control valve actuator. Currently the turbine uses the following (also sketch attached):
- Valtek Mark I size 25 valve with Valtek VL double action piston actuator.
- Air supply 25 psig
- Failsafe return spring 1450 N when compressed.
- Total required travel 0.63 in (16 mm)

The change we´re proposing is to remove the pneumatic actuator and hook up a hydraulic one, but I´m having trouble determining the required force this new actuator should be capable of doing.
With 65 psig of air only on the piston rod side, the maximum force side would be F = A · P = 148.87 cm2 · 65 psig · [units conversion factor] = 6674 N
For a single action piston, the needed force would be [Total force] = [spring max force] + [friction] + [whatever the steam flow generates]
However, this is a double action piston, so the positioner also gives pressure on the non-rod side.
In summary, how would you estimate the maximum NET FORCE on the valve stem that this actuator is doing?